The present invention is directed to a display apparatus with an addressable matrix display device of the type having pixels which are driven to produce light. In the present document, such a display device will be called an addressable matrix display device with luminescent pixels. Such display devices are to be distinguished from display devices which employ luminescent pixels but not an addressable matrix, such as cathode ray tubes. Such display devices are also to be distinguished from display devices which employ an addressable matrix of light valves to modulate light from a source that illuminates the light valves. Examples here include liquid crystal display devices and digital micro-mirror devices.
Several families of addressable matrix display devices with luminescent pixels have been proposed. These include LED arrays, plasma display panels, electroluminescent panels, and field-emission display devices. In each case, the display device has an addressable matrix of light-generating means.
A summary of the current (1998) state of the art in field-emissions display devices was presented in an article by Babu R. Chalamala et al, "Fed Up With Fat Tubes," at pages 42-51 of the April 1998 issue of IEEE Spectrum. An example of a conventional field-emission display device 10 is shown in FIG. 1. It includes a base plate 12 which supports a number of parallel cathode electrodes 14 (only a few of which are shown). A number of parallel gate electrodes 16 (only a few of which are shown) cross the cathode electrodes 14 and are spaced apart from them by a small gap. The gate electrodes 16 have holes 18 to permit passage of electrons. Red phosphor stripes 20, green phosphor stripes 22, and blue phosphor stripes 24 are deposited on a transparent anode electrode 26 made, for example, from ITO. The assembly is disposed in a vacuum envelope (not shown).
The cathode electrodes 14 emit electrons when a suitable potential is applied between the cathode electrodes 14 and the gate electrodes 16. Unlike the emission process in a cathode ray tube, which employs a heater, the cathode electrodes 14 are cold cathodes which are stimulated to emit electrons when a strong electric field is present. Several approaches are known which provide good cathode electrodes. In FIG. 1, so-called Spindt tips 28 are included in the cathode electrodes 14, and project upward at the locations of the holes 18 to provide enhanced electron emissions. Typically, a plurality of Spindt tips are employed in each pixel. For example, in FIG. 1, the red pixel 30 is shown as having four Spindt tips 18 which cooperate with four holes 28 to generate current for illuminating the adjacent portion of red phosphor stripe 20. A green pixel and a blue pixel are provided adjacent the pixel 30, along the same gate electrodes 16, to form a three-pixel group having all three primary colors.
There is minimal electron emission from the cathode electrodes until they are exposed to an electric field that is higher than a threshold value. When the field strength is higher than the threshold, the number of electrodes emitted per unit of time (that is, the current) increases as the strength of the electric field increases. FIG. 2 illustrates a typical example, with the horizontal axis representing the voltage between a cathode electrode 14 and a gate electrode 16 (which are closely spaced, so that a potential between them of only 45 volts is sufficient to produce an electric field that is larger than the threshold) and the vertical axis represents current in microamperes.
The electrons emitted by the cathode electrodes 14 under the influence of the gate electrodes 16 are accelerated toward the phosphor stripes 20-24 by a positive voltage that is placed on the anode 26. The higher the anode voltage, the greater the acceleration and, for a given cathode current, the brighter the light produced by the phosphors. The phosphors also glow more brightly when the cathode current is increased. However, increasing the cathode current does not appreciably increase the brightness of the phosphor glow after the current reaches a so-called current saturation level. Furthermore, too high a cathode current degrades the phosphors.
Among other addressable matrix-type display devices that are known are display devices which employ an array of light valves to spatially modulate light that shines on the array. The light valves may have variable optical densities or attenuations properties that are electrically controlled so as to determine the percentage of the impinging light that passes through each light valve. An example is a twisted nematic liquid crystal display device. Other spatial light modulators employ bi-stable light valves, meaning that they are either on or off. Examples here include digital micro-mirror devices and ferroelectric liquid crystal display devices. Since the light valves are bi-stable, rather than having continuously variable attenuation, special measures must be taken in order to provide a gray scale in a display apparatus which includes such a display device. For a display apparatus with a digital micro-mirror device, a gray scale can be achieved by using a pulse width modulation scheme in which the length of time in which the micro-mirrors are in their ON position is controlled, as is explained in U.S. Pat. No. 5,452,024 and in an article entitled "Mirrors on a Chip" that was published in the November 1993 edition of IEEE Spectrum at pages 27-31 by Jack M. Younse. For a display apparatus with a ferroelectric display device, a gray scale can be achieved by controlling the intensity or duration of the back-lighting on the basis of the rank or significance of the bits that are being displayed, as is disclosed in U.S. Pat. Nos. 5,122,791 and 5,416,496. Controlling the intensity of the impinging light on the basis of the rank of the displayed bits can also be used in a display apparatus with a digital micro-mirror device to control the level of gray that is displayed at each pixel, as is explained in Applicant's co-pending applications Ser. Nos. 08/381,156 and 09/063,364, filed respectively on Jan. 31, 1995 and Apr. 21, 1998.